A head/disk tester is an instrument that is used for testing the characteristics of magnetic heads and disks, such as a signal-to-noise ratio, track profile, etc. The tester should simulate those motions of the head with respect to the disk that occur in an actual hard disk drive during operation. A tester comprises a mechanical component, commonly referred to as a spinstand, that performs movements of the head with respect to the disk, and an electronic component that is responsible for measurement, calculation, and analysis of the measured signal.
Examples of prior art spinstands for a head and disk tester include the Guzik V2002 XY-positioning spinstand and the Guzik 5-1701B Micro Positioning spinstand, both of which are available from the assignee of the present disclosure, Guzik Technical Enterprises, 2443 Wyandotte Street, Mountain View, Calif. 94043, USA (www.guzik.com).
For testing, a magnetic read/write head is usually incorporated into a structure known as a head gimbal assembly (HGA). An exemplary HGA 1 is shown in FIG. 1. The basic components of an HGA 1 are a head 2, an elongated load beam 4 bearing the head at a distal end, a tooling hole 6, a base plate 8 having a planar mounting surface, a boss hole 10 with an angled surface 10a, and an elongated flex circuit support sheet element 12 with an array of electrically conductive pads 18 at its distal end. The boss hole 10 passes through base plate 8 and is characterized by a radius R about an HGA mounting axis perpendicular to the planar mounting surface of base plate 8. The head 2 is disposed along an “axis of symmetry” extending along the load beam 4 from the center point CP of the boss hole 10 to head 2. The array of electrical contacts 18 is disposed along a “flex axis” extending from the center point CP of the boss hole 10 to array 18. The tooling hole 6 is on load beam 4, between the boss hole 10 and head 2. The boss hole 10 and the tooling hole 6 (sometimes) are used, in the prior art, for orientation of the HGA in a plane transverse to an HGA mounting axis. The angled surface 10a of the boss hole 10 is used for clamping the HGA to an HGA support assembly associated with a spinstand. The flex circuit sheet element 12 is used to support electrical connections of the head of the HGA, by way of pads 18, to an external head preamplifier (not shown). Generally, the base plate 8 and load beam 4 are relatively stiff compared to the flex circuit sheet element 12.
In order to test a head with a spinstand, an HGA is loaded to an HGA support assembly associated with the tester. The HGA is mechanically coupled to a corresponding component of the spinstand, and electrically connected to spinstand preamplifiers which provide test signals and receive back response signals from the head under test. To make these operations possible, an alignment of the HGA relative to the spinstand is carried out. In FIGS. 2-5, and the text below, HGAs 1 are illustrated only by their load beam (which is identified with the HGA's reference numeral 1); the other elements of the HGAs, and portions of a tester for applying test signals to and analyzing responses from the HGAs, although present in the illustrated and described HGAs, are not shown in the figures.
A major step forward in improvement of the qualitative parameters of spinstands was made in the U.S. Pat. No. 8,169,750. In that patent, a spinstand is described that includes a base, a Y coarse positioning stage that moves in a Y direction and a X coarse positioning stage that is movable in a X direction, where the X and Y directions are defined with respect to an x-y-z Cartesian coordinate system. The X coarse positioning stage is coupled to the Y coarse positioning stage by a linear bearing. An X precision (or fine motion) positioning stage, movable in an X direction, is mounted on the X coarse positioning stage. The X precision positioning stage comprises a piezo-electric actuator and a parallelogram flexure assembly (or parallelogram) having a base element rigidly mounted to the X coarse positioning stage, and a movable (with respect to the X coarse positioning stage) element maintained parallel to the base element by a pair of equal length end elements flexure-coupled to the base element and the movable element. These components make possible movement of the X precision positioning stage in the X direction. Position feedback for X precision positioning stage is provided by a displacement sensor that comprises a linear glass scale mounted to the moving element of the parallelogram disposed opposite to an optical reader mounted to the X coarse positioning stage. The sensor carries out measurement of the displacement of the moving element of the parallelogram relative to the X coarse positioning stage, and produces in that way information of the read/write head position.
The spinstand also includes a removable HGA-bearing-only cartridge that enables magnetic head and disk testing using different magnetic heads with the possibility of quick installation of a head and quick dismount of the head from the cartridge without special tools or alignment procedures. The cartridge is rigidly coupled to the moving element of the parallelogram, which is located on the X precision positioning stage. A head gimbal assembly HGA mounts on the cartridge. A read/write head is a part of HGA. To move the read/write head, a piezo-electric actuator mounted on the X precision stage drives the parallelogram to move the cartridge, which typically has a mass that exceeds 500 grams. These large mass results in relatively slow movement and a relatively low mechanical bandwidth compared to generally desired movements and bandwidths.
A substantial improvement of spinstand parameters was achieved in the cited '750 patent by introducing an additional piezo-electric actuator that acts on the head-mounting unit. Such an actuator is mounted on the base of the cartridge and moves the relatively low mass head-mounting unit (compared to the above described head-mounting unit) together with the read/write head directly. The reduction of the mass of the head-mounting unit leads to corresponding widening of the mechanical bandwidth and an increase in movement speed. As a result, the accuracy of the read back process is improved, since the wider bandwidth extends the ability of the head to follow applied servo commands.
Along with the noted positive properties applicable to operation in a servo read mode, a cartridge and spinstand according to the U.S. Pat. No. 8,169,750 has a serious drawback that limits positioning accuracy in a servo write mode. In an embodiment described in the '750 patent, the position of read/write head in the servo write mode is changed by a piezo actuator acting on the parallelogram of the X precision positioning stage, and the change in position is measured by a glass scale on that stage. The piezo actuator on the cartridge is not used in that measurement of position change. For this reason, the movement is slow and servo writing takes a relatively long time. Moreover, the distance between the HGA and glass scale is relatively large and variations of that distance due to temperature changes, vibrations etc. create positioning errors during servo writing. For these reasons, the systems of U.S. Pat. No. 8,169,750 did not improve spinstand performance in a servo write mode.
Another disadvantage of prior art spinstands, is connected to vibrational movement of the components that support the HGA. To move the head-mounting unit, the piezo-electric actuator acts on that unit with a force F directed along a displacement axis. According to a law of physics, the force F evokes a counteractive force R that is applied to the actuator and through the actuator to the cartridge itself. At each positioning operation, the counteractive force R causes oscillations of the cartridge base together with the adjacent components. For this reason, measurement of the HGA position cannot be initiated until the system has settled down to a substantially stable condition. Thus occurrence of the counteractive force R brings about degradation of the positioning accuracy and increase of the settling time.
To reduce vibration of the cartridge base it was proposed in U.S. Pat. No. 6,006,614, issued on Dec. 28, 1999, assigned to the assignee of this disclosure, to complement the head-mounting unit by a counterweight. According to that proposal, two piezo-electric actuators are mounted on a precision positioner. During positioning of an HGA, the two actuators act simultaneously on the head-mounting unit and on the counterweight, with equal magnitude forces in opposing directions along the displacement axis. In this case, two counteraction forces appear where the forces are of equal magnitude and are oppositely directed. As a result, the composite force applied to the positioner turns out to be of negligible magnitude, so that the cause of vibrations is eliminated or reduced significantly.
However, there is a necessary condition for effective suppression of system vibrations by counterweight introduction: the product of the mass of the head-mounting unit by the expansion coefficient of the actuator that advances the head-mounting unit, should equal the product of the mass of the counterweight by the expansion coefficient of the actuator that advances the counterweight. The practice of using a positioner with an added counterweight showed that it is was difficult to achieve sufficiently accurate fulfillment of this condition, especially for the case of high frequency vibration.
More importantly, it has been determined that the introduction of counterweight suppresses vibration in the cartridge base only, and it does not affect the vibration of a head-mounting unit itself and of the counterweight. Therefore, the introduction of a counterweight does not solve the problem of vibration completely.
The goal of the current invention is to effect an HGA positioning cartridge that eliminates the above-outlined disadvantages and thereby improves the head positioning accuracy with simultaneous reduction of the settling time.